The nagging feeling that something has changed became more than a feeling this week, and I'm not talking about the classic rock song by Boston . Yes, the sneaking suspicion the postwar rally had run out of momentum, at least temporarily, became a reality this week as major averages fell uniformly. More dramatically, Treasury prices plummeted and yields jumped after the Federal Reserve's midweek decision to lower rates by 25 basis points. For the week, the Dow Jones Industrial Average fell 2.3%, the S&P 500 shed 1.9%, and the Nasdaq Composite lost 1.2%. The failure of Monday's merger announcement between Idec Pharmaceuticals ( IDPH) and Biogen ( BGEN) to rekindle strength in the biotech sector, much less the broader market, was a clear signal the rally's worm had turned. Also troublingly, from a technical perspective, both Wednesday and Friday were reversal days, whereby major averages established higher highs and lower lows than in the prior session. The pattern has more significance if there's an accompanying volume surge -- not the case in either instance -- but some believe the reversal days are emblematic of a market running into trouble. Included among them is Charles Biderman, president of TrimTabs.com Investment Research in Santa Rosa, Calif. At a luncheon at Moose's restaurant in San Francisco on Thursday afternoon, Biderman laid out the case for why he believes the market is exhibiting signs of a "classic top." The signs include heavy corporate selling, renewed optimism among individual investors (the American Association of Individual Investors this week reported that bullishness among its members rose to 71.4% while bearishness plunged to 8.6%), and the increased use of margin debt, including that used by corporations. General Motors ( GM) will use some of the proceeds of its record $17.6 billion corporate bond offering to replenish its pension fund, which very likely will invest much of that money in equities.